1. Field of the Invention
The invention relates to piezoelectric hydrophones with increased sensitivity.
Piezoelectric hydrophones generally include one or more sensitive elements each formed of a substrate with piezoelectric properties and two electrodes disposed on each side.
2. Description of the Prior Art
The substrate may be in the form of ceramic disks of relatively small dimensions, included in a rigid case having one or more faces transparent to the acoustic waves. U.S. Pat. No. 3,763,464 shows the construction of a piezoelectric hydrophone in which one, and preferably two ceramic disks, each provided with its electrodes, are fixed to two flexible plates facing each other. One at least on the plates is provided with a thicker rigid peripheral part bearing against the other.
The substrate may also be in the form of ribbons or flexible plates made from synthetic plastic material on which treatment confers piezoelectric properties. Materials are generally used such as PVDF (polyvinylidene fluoride), polyethylene, PTFE (polytetrafluoride ethylene) etc.
The flexible sensitive elements are very often used in the form of ribbons or strips disposed on a flat support or else would in a spiral about a cylindrical core. They are generally combined in pairs. By disposing them on each side of a support with opposite polarities and, by connecting them electrically, the parasitic voltages caused by bending of the support and acceleration are compensated for.
Using flexible strips, continuous hydrophones may be formed of relatively great length which provide wave number filtering. Incorporated in seismic streamers towed by a ship, they filter out certain particular parasitic noises and facilitate reception of the signals reflected by discontinuities of the submerged subsoil, in response to seismic waves emitted by a towed source.
Sensitive elements of this type are described in the French patent No. 2 145 099 to which correspond the U.S. Pat. No. 2,798,474, and the French patent application EN. 86/09.880.
The sensitive elements are very often formed from ribbons of small thickness, of the order of a few tens of .mu.m, whose sensitivity is relatively low, generally less than 1V/bar (1 bar=10.sup.5 Pa). For increasing the sensitivity of the sensors formed from such sensitive elements, several of them must be combined electrically, which increase the cost price and complicates the manufacture.